13.7.2.3 CHECKSUM TABLE Syntax

CHECKSUM TABLE tbl_name [, tbl_name] ... [ QUICK | EXTENDED ]

CHECKSUM TABLE reports a
checksum for the contents
of a table. During the checksum operation, the table is locked
with a read lock for InnoDB and
MyISAM. You can use this statement to verify
that the contents are the same before and after a backup,
rollback, or other operation that is intended to put the data
back to a known state. This statement requires the
SELECT privilege for the table.

Performance Considerations

By default, the entire table is read row by row and the checksum
is calculated. For large tables, this could take a long time,
thus you would only perform this operation occasionally. This
row-by-row calculation is what you get with the
EXTENDED clause, with
InnoDB and all other storage engines other
than MyISAM, and with
MyISAM tables not created with the
CHECKSUM=1 clause.

For MyISAM tables created with the
CHECKSUM=1 clause, CHECKSUM
TABLE or CHECKSUM TABLE ... QUICK
returns the “live” table checksum that can be
returned very fast. If the table does not meet all these
conditions, the QUICK method returns
NULL. See Section 13.1.17, “CREATE TABLE Syntax” for
the syntax of the CHECKSUM clause.

For a nonexistent table, CHECKSUM
TABLE returns NULL and generates a
warning.

The checksum value depends on the table row format. If the row
format changes, the checksum also changes. For example, the
storage format for VARCHAR
changed between MySQL 4.1 and 5.0, so if a 4.1 table is upgraded
to MySQL 5.0, the checksum value may change.

Important

If the checksums for two tables are different, then it is
almost certain that the tables are different in some way.
However, because the hashing function used by
CHECKSUM TABLE is not
guaranteed to be collision-free, there is a slight chance that
two tables which are not identical can produce the same
checksum.